<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>M</mml:mi><mml:mrow><mml:mi>n</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msub><mml:mi>A</mml:mi><mml:msub><mml:mi>X</mml:mi><mml:mi>n</mml:mi></mml:msub></mml:mrow></mml:math>phases in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:mtext>−</mml:mtext><mml:mi mathvariant="normal">Si</mml:mi><mml:mtext>−</mml:mtext><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>system studied by thin-film synthesis and<i>ab initio</i>calculations

Thin films of ${M}_{n+1}A{X}_{n}$ layered compounds in the $\mathrm{Ti}\text{\ensuremath{-}}\mathrm{Si}\text{\ensuremath{-}}\mathrm{C}$ system were deposited on $\mathrm{MgO}(111)$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ substrates held at $900\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ using dc magnetron sputtering from elemental targets of $\mathrm{Ti}$, $\mathrm{Si}$, and $\mathrm{C}$. We report on single-crystal and epitaxial deposition of ${\mathrm{Ti}}_{3}{\mathrm{SiC}}_{2}$ (the previously reported $MAX$ phase in the $\mathrm{Ti}\text{\ensuremath{-}}\mathrm{Si}\text{\ensuremath{-}}\mathrm{C}$ system), a previously unknown $MAX$ phase ${\mathrm{Ti}}_{4}{\mathrm{SiC}}_{3}$ and another type of structure having the stoichiometry of ${\mathrm{Ti}}_{5}{\mathrm{Si}}_{2}{\mathrm{C}}_{3}$ and ${\mathrm{Ti}}_{7}{\mathrm{Si}}_{2}{\mathrm{C}}_{5}$. The latter two structures can be viewed as an intergrowth of 2 and 3 or 3 and 4 $M$ layers between each $A$ layer. In addition, epitaxial films of ${\mathrm{Ti}}_{5}{\mathrm{Si}}_{3}{\mathrm{C}}_{\mathrm{x}}$ were deposited and ${\mathrm{Ti}}_{5}{\mathrm{Si}}_{4}$ is also observed. First-principles calculations, based on density functional theory (DFT) of ${\mathrm{Ti}}_{n+1}{\mathrm{SiC}}_{n}$ for $n=1$,2,3,4 and the observed intergrown ${\mathrm{Ti}}_{5}{\mathrm{Si}}_{2}{\mathrm{C}}_{3}$ and ${\mathrm{Ti}}_{7}{\mathrm{Si}}_{2}{\mathrm{C}}_{5}$ structures show that the calculated difference in cohesive energy between the $MAX$ phases reported here and competing phases ($\mathrm{TiC}$, ${\mathrm{Ti}}_{3}{\mathrm{SiC}}_{2}$, ${\mathrm{TiSi}}_{2}$, and ${\mathrm{Ti}}_{5}{\mathrm{Si}}_{3}$) are very small. This suggests that the observed ${\mathrm{Ti}}_{5}{\mathrm{Si}}_{2}{\mathrm{C}}_{3}$ and ${\mathrm{Ti}}_{7}{\mathrm{Si}}_{2}{\mathrm{C}}_{5}$ structures at least should be considered as metastable phases. The calculations show that the energy required for insertion of a $\mathrm{Si}$ layer in the $\mathrm{TiC}$ matrix is independent of how close the $\mathrm{Si}$ layers are stacked. Hardness and electrical properties can be related to the number of $\mathrm{Si}$ layers per $\mathrm{Ti}$ layer. This opens up for designed thin film structures the possibility to tune properties.

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